Transmission line



June 4, 1957 IIIE S J. D. GIULIE ETAL 2,794,848

TRANSMISSION LINE Filed July 14, 1955 IIE E w -zv B Y ,4 member of #1: firm nited States Patent TRANSMISSION LINE Application July 14, 1953, Serial No. 367,916 1 Claim. (Cl. 174-113) This invention relates to an open wire transmission line and the method of making the same. The transmission line of the present invention is particularly adapted for the carrying of ultra-high frequency electric currents, and as such finds its most common application as television lead-in wire, although many other uses will be apparent to those skilled in the art.

In recent years, it has been common practice to carry both high frequency and very high frequency electric currents on open wire transmission lines. One typical line consists of two No. 18 wires spaced about 1 apart and held in place by a series of thermoplastic spacers such as set forth in our patent application Serial No. 280,416, filed April 3, 1952. Such transmission lines are very effective for carrying very high frequency current, but have a higher attenuation when one attempts to carry ultra-high frequency currents on the lines. One reason for this is that the spacing of the wires becomes an appreciable fraction of a Wave length so that the wires tend to radiate. Further, since the dielectric spacers occupy the region of the most intense field, i. e., directly between the wires, the spacers introduce a further degree of attenuation.

If one attempts to remedy this situation by merely using shorter spacers, the spacers still occupy the most intense field and have a short leakage path.

It is an object of the present invention to provide a transmission line for the transmission of ultra-high frequency electric currents wherein the dielectric is substantially entirely air and only an inconsequential amount of solid dielectric appears between the two parallel lines.

Another object of this invention is to provide an air insulated transmission line having spacers wherein the spacers have a leakage path substantially longer than the distance between the two transmission wires.

Still another object of this invention is to provide a simple and effective method of making an open wire transmission line having the above enumerated desirable characteristics.

Referring to the drawings:

Figure 1 is a perspective view of a transmission line made in accordance with the present invention.

Figure 2 is a cross-section of one of the spacers of the transmission line of Figure 1.

Figures 3 through 6 illustrate various embodiments of the spacers which may be used for transmission lines in accordance with the present invention.

Referring to the drawings by reference characters, there is shown a transmission line having two parallel wires 8, which are held in spaced relationship by a spacer member 12. The wires 8 may be of any suitable material such as iron or copper and preferably have plated thereon a thin layer of a highly conducting material 14, such as silver. Tubing may be used instead of wire. Over the silver plating is preferably an outer coating of a protective material 16. The protective material 16 ordinarily serves to insulate the wire but, more important, makes the wire 2,794,848 Patented June 4, 1957 '2- corrosion-resistant. In this manner, the effects of weathering on the transmission lines are greatly lessened and the transmission line retains its original low attenuation for a long period of time, even when exposed to the elements. One particularly suitable coating 16 consists of Formvar, which is a polyvinylmethylal resin.

The spacers 12 may be of any suitable insulating material such as plastic, ceramic or glass. However, because of the ease of fabrication, coupled with good electrical properties, we prefer to use a thermoplastic plastic such as polystyrene. Polystyrene is particularly suitable since it lends itself to an easy method of fabricating the line, as is set forth in more detail below.

A preferred method of making the transmission line of the present invention is to first form an open wire transmission line having a spacing about twice the desired ultimate spacing, using straight spacers of a thermoplastic material such as polystyrene. The line which is used as a starting material may suitably be a transmission line having straight spacers, as illustrated in our pending patent application, Serial No. 280,416. After forming a transmission line using straight, thermoplastic spacers, the center of each of the plastic spacers may be heated until the central portion becomes somewhat softened. Each insulator may then be bent to the shape shown in Figure 2. Upon cooling, the plastic insulators become rigid so that the line is left in the condition shown in Figure 1.

In one typical operation, polystyrene rod was employed as a spacing material, said rod having a diameter of A". The rod was cut into sections each about 1% long and No. 18 copper wires, which had been silver-plated and coated with Formvar enamel, were embedded in the ends of the spacer so that the spacing between the wires was about 1". The spacers were located about 6" apart on the wires. The centers of the spacers were then heated and the spacers bent into a U-shape, as illustrated in Figure 2, so that the wires were brought to a separation of about /2".

A line formed in this manner has a surge impedance of slightly less than 400 ohms, has no substantial amount of dielectric, other than air, in the high intensity field between the wires, and the insulators have a much longer leakage path than would be obtained if straight insulators were used.

The spacing of the insulators along the wire is not critical and distances of from 6" to 12 have been employed. It is often advantageous to not space the insulators at regular intervals, but to place them irregularly along the wire in orderthat resonances may be avoided.

Although it is generally preferred to make the spacers in a U-shape, as is illustrated in Figures 1 and 2, the spacers may be made in a variety of forms, but in each of the forms, the leakage path between the wires is much longer than if straight insulators were employed and there is no substantial amount of insulation between the wires in the high field portion.

Thus, in Figure 3, an insulator 18 is employed which is made in a rectangular U-shape.

In Figure 4, an insulator 20 is illustrated which is made in the form of a doughnut, the sides being slightly elongated.

In Figure 5, an insulator is illustrated made in the form of a sharp V.

In Figure 6, an insulator is illustrated wherein the insulator is in the shape of a U, with the sides of the U pressed outwardly.

I claim:

An open Wire transmission line comprising a pair of parallel spaced wires, said spaced Wires being held in a spaced relationship by a series of spacers thereon, said spacers being of a U-shaped configuration wherein the 3 .wires are gripped at the ends. of the U, the distancebetween said spacers along said transmission line being rela tively large as compared with the distance over which said spacers are in contact with the transmission line.

References Cited in the fiIe of this patent UNITED STATES PATENTS 2,158,496 George May 16, 1939 4 2,428,052 Touraton Sept. 30, 1947 2,543,696 Krueger Feb. 27, 1951 FOREIGN PATENTS 947,544 France Jan. 10, 1949 665,239 Great Britain Jan. 16, 1952 OTHER REFERENCES 

